This invention is directed to radiography and in particular to image storage assemblies that are useful for oncology portal imaging. It also relates to radiation image recording and reproducing method. Thus, this invention is useful in portal radiography.
In conventional medical diagnostic imaging the object is to obtain an image of a patient""s internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifing screens for imagewise exposure. About 5% or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element.
Examples of radiographic element constructions for medical diagnostic purposes are provided by U.S. Pat. No. 4,425,425 (Abbott et al.) and U.S. Pat. No. 4,425,426 (Abbott et al.), U.S. Pat. No. 4,414,310 (Dickerson), U.S. Pat. No. 4,803,150 (Kelly et al.), U.S. Pat. No. 4,900,652 (Kelly et al.), U.S. Pat. No. 5,252,442 (Tsaur et al.), and Research Disclosure, Vol. 184, August 1979, Item 18431.
Radiation oncology is a field of radiology relating to the treatment of cancers using high energy X-radiation. This treatment is also known as teletherapy, using powerful, high-energy X-radiation machines (often linear accelerators) or cobalt (60) units to exposure the cancerous tissues (tumor). The goal of such treatment is to cure the patient by selectively killing the cancer while minimizing damage to surrounding healthy tissues.
Such treatment is commonly carried out using high energy X-radiation, 4 to 25 MVp. The X-radiation beams are very carefully mapped for intensity and energy. The patient is carefully imaged using a conventional diagnostic X-radiation unit, a CT scanner, and/or an MRI scanner to accurately locate the various tissues (healthy and cancerous) in the patient. With full knowledge of the treatment beam and the patient""s anatomy, a dosimetrist determines where and for how long the treatment X-radiation will be directed, and predicts the radiation dose to the patient.
Usually, this causes some healthy tissues to be overexposed. To reduce this effect, the dosimetrist specifies the shape of the beam that will be controlled by lead blockers at the source or xe2x80x9cportxe2x80x9d of the treatment device. This effectively acts as a substantially opaque block in front of parts of the patient""s body that absorbs X-radiation that would impact healthy tissues.
To this end, three distinct types of imaging are carried out in radiation oncology.
The first type of imaging is called xe2x80x9csimulationxe2x80x9d. In this procedure, the patient is carefully imaged using a conventional diagnostic X-radiation unit, a conventional radiographic imaging film system, a storage phosphor system, or a digital system. In addition, a CT scanner and/or MRI scanner may be used to accurately locate the patient""s anatomy. These procedures are essentially like those used in diagnostic radiography. They are carried out using from 80 to 150 kVp with low doses of radiation. These images provide detailed information on the patient""s anatomy, and the location of the cancer relative to other body parts.
From the simulation images and/or CT/MRI data, a dosimetrist can determine where and for how long the treatment X-radiation should be directed. The dosimetrist uses a computer to predict the X-radiation dose for the patient. Usually this leads to some normal tissues being overexposed. The dosimetrist will introduce one or more xe2x80x9cblocksxe2x80x9d or lead shields to block X-radiation from normal anatomy. Alternatively, where available, the dosimetrist can shape the beam by specifying the positions for a multi-leaf collimator (MLC).
To determine and document that a treatment radiation beam is accurately aimed and is effectively killing the cancerous tissues, two other types of imaging are carried out during the course of the treatment. xe2x80x9cPortal radiographyxe2x80x9d is generally the term used to describe such imaging. The first type of portal imaging is known as xe2x80x9clocalizationxe2x80x9d imaging in which the portal radiographic film is briefly exposed to the X-radiation passing through the patient with the lead shields removed and then with the lead shields in place. Exposure without the lead shields provides a faint image of anatomical features that can be used as orientation references near the targeted feature while the exposure with the lead shields superimposes a second image of the port area. This process insures that the lead shields are in the correct location relative to the patient""s healthy tissues. Both exposures are made using a fraction of the total treatment dose, usually 1 to 4 monitor units out of a total dose of 45-150 monitor units. Thus, the patient receives less than 20 RAD""s of radiation.
If the patient and lead shields are accurately positioned relative to each other, the therapy treatment is carried out using a killing dose of X-radiation administered through the port. The patient typically receives from 50 to 300 RAD""s during this treatment.
A second, less common form of portal radiography is known as xe2x80x9cverificationxe2x80x9d imaging to verify the location of the cell-killing exposure. The purpose of this imaging is to record enough anatomical information to confirm that the cell-killing exposure was properly aligned with the targeted tissue. The imaging film/cassette assembly is kept in place behind the patient for the full duration of the treatment. Verification films have only a single field (the lead shields are in place) and are generally imaged at intervals during the treatment regime that may last for weeks.
Portal radiographic imaging film, assembly and methods are described, for example, in U.S. Pat. No. 5,871,892 (Dickerson et al.) in which the same type of radiographic element can be used for both localization and portal imaging.
A radiographic phosphor panel contains a layer of phosphor, a crystalline material that responds to X-radiation on an imagewise basis. Radiographic phosphor panels can be classified, based on the type of phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels and are generally used to generate visible light upon exposure to provide an image in radiographic silver halide materials.
Storage phosphor panels comprise storage phosphors that have the capability of storing latent X-radiation images for later emission. Storage phosphors are distinguishable from the phosphors used in intensifying screens because the intensifying screen phosphors cannot store latent images for later emission. Rather, they immediately release or emit light upon irradiation. Various storage phosphors are described, for example in U.S. Pat. No. 4,368,390 (Takahashi et al.) and U.S. Pat. No. 5,464,568 (Bringley et al.).
Early use of storage phosphor systems for portal imaging used no metal converter screen. However, this adversely affects image quality as pointed out in several publications [for example, Wilenzink et al., Med. Phys., 14(3), 1987, pp. 389-392, and David et al., Med. Phys., 16(1), 1989, pp. 132-136].
Subsequent teaching in this art suggests that 1 mm copper metal plate would enhance contrast and image quality [for example, Weiser et al., Med. Phys. 17(1), 1990, pp. 122-125, and Roehrig et al., SPIE, 1231, 1990, pp. 492-497]. Soon thereafter, aluminum, copper, tantalum, and lead metal plates were considered with storage phosphor screens [Barnea et al., Med. Phys., 18(3), 1991, pp.432-438]. The tantalum metal plates were 0.4 mm thick to reach equilibrium at 6 MV. Thus, the conventional understanding in the art is that even storage phosphor panels require relatively thick metal screens to improve image quality. However, the weight of such image storage assemblies is considerable and creates a problem for users in the medical imaging community.
Since the earliest teaching about the need for metal screens in image storage assemblies, the thickness of the metal screens has been set at 1 mm or more when copper is used and at 0.6 mm when lead is used. It was consistently believed that thick metal screens were required to avoid overexposure especially for portal imaging.
Conventional image storage assemblies are illustrated in FIG. 1 (described in more detail below). A commercial imaging assembly of this type is used with the commercially available KODAK ACR 2000 system (Eastman Kodak Company).
Thus, conventional image storage assemblies provide the desired high contrast images, but because of the thick metal screens used to provide the desired imaging features, they are very heavy and difficult and unsafe to carry throughout medical facilities. Medical users have tolerated this disadvantage because it has been believed that the thick metal plates are necessary for desired imaging properties. We have found that this is not the case and it is to this problem that the present invention is directed.
This invention provides a radiation image recording and reproducing method comprising the steps of:
A) exposing a storage phosphor in a radiographic image storage assembly to reflected or transmitted radiation of a first wavelength to store the radiation within the storage phosphor,
the radiographic image storage assembly comprising a metal screen that is positioned adjacent to a storage phosphor screen comprising the storage phosphor, the metal screen having a thickness of from about 0.1 to 0.75 mm when composed of copper or a thickness of from about 0.05 to 0.40 mm when composed of lead, and
the radiographic image storage assembly positioned in relation to the exposing radiation of the first wavelength such that the exposing radiation passes through the metal screen before the storage phosphor screen,
B) exposing the storage phosphor to radiation of a second wavelength to release the stored radiation as light emission of a third wavelength, the second wavelength radiation being directed through the metal screen before it reaches the storage phosphor screen, and
C) detecting the emitted light of the third wavelength.
The present invention provides a means for portal imaging using a light-weight image storage assembly (or cassette) that has the desired imaging properties. It was a surprise to us that the thickness of the metal screen could be significantly reduced from 1 mm without any significant loss in imaging properties or photographic speed. This invention will allow users to achieve portal imaging with lightweight portal imaging systems that are safer and easier to handle.
The image storage assembly of this invention can be used in portal imaging whereby the first imaging radiation is directed through the thin metal screen before it reaches the storage phosphor. Thus, the thin metal screen is considered to be in xe2x80x9cfrontxe2x80x9d of the storage phosphor screen for portal imaging. The storage phosphor can then be stimulated by directing second radiation through the thin metal screen before it reaches the storage phosphor. Thus, the conventional thick metal screen used in portal imaging is replaced with the thin metal screen described herein, resulting in a very lightweight imaging assembly.